17th June 2018
Our advisor, GP Imogen Staveley sets out what prenatal screening involves, and what you need to know about the different tests.
A first time mum came to me this week overwhelmed by her options around screening tests for herself and her unborn baby. She had been bamboozled by the options particularly around screening for Down’s syndrome.
Down’s syndrome is the most common genetic condition to be found in humans and the genetics behind it are relatively well understood, which is why screening has been made available for it. Babies born with Down’s syndrome generally have a learning disability and an array of physical attributes characteristic of the condition as well as being more likely to get other physical complications such as heart problems. The life expectancy of these babies is reduced although with health advances this is increasing all the time.
The combined test is not diagnostic; it gives a woman a probability of her baby being at risk of having Down’s syndrome. If she is found to be high risk she is then offered non-invasive prenatal testing (NIPT). This is a relatively new option on the NHS. This test is also not diagnostic but its false positive rate is far less than for the combined test. If women are found on this test to be high risk of having a baby with Down’s syndrome they are then offered a diagnostic test: amniocentesis or chorionic villus sampling. You can read more about these here: http://bit.ly/amnioNHS and http://bit.ly/CVSNHS. They are not without risk.
Women choose to have screening +/- diagnostic tests for different reasons. Some want to know if they are going to have a baby with Down’s syndrome so that they can prepare for this. Others want to know so that they can have the option to terminate the pregnancy. It is a woman’s right to choose and it is really important she understands her choice every step of the way.
A woman can pay privately to go straight for the NIPT as early as 9 weeks depending on which type of NIPT she chooses. You can already see why she was in a dilemma. Firstly the woman had to understand Down’s syndrome, then choose from the tests available and finally consider what she would do should she find out she was high risk of having a baby with Down’s syndrome at each stage of testing.
Here are some resources for understanding Down’s syndrome:
Here is a resource to help you as a mother or couple decide what to do should you find your tests show a pregnancy high risk for Down’s syndrome: http://www.arc-uk.org/for-parents/decision-making
Here is a resource about pre-natal screening on the NHS:
This blog aims to help women to understand a bit more about screening options during early pregnancy for Down’s syndrome. It is a complex area and it is recommended that women discuss their options with their partner/friends/family and their midwife/doctor to come to a decision they are happy with. No two women are the same and this is an individual decision but there are plenty of people out there to support women to make it. I hope this blog helps.
Is it worth paying privately for the NIPT test?
This blog also sets out arguments for and against having the NHS combined test or paying privately to go straight for NIPT. Clearly the private option will only be available to those that can fund it and the cost is in the range of ~£350 to ~£600.
It will help a woman decide which test she wants if she knows what she would do should she discover she is having a baby with a high likelihood of having Down’s syndrome. There is more information on this in my last blog: bit.ly/Downsprenatal
Timelines also help a woman decide. If she goes privately she can choose to have NIPT as early as 9 weeks although for most tests being 10 weeks or more pregnant is usually recommended. This would mean that if she gets a high-risk result she is able to have diagnostic testing earlier and then choose to keep the baby or have a termination earlier in the pregnancy. It also reduces the steps in the process e.g. she doesn’t have to wait for the combined test results and then wait for the NIPT.
Finally, women need to understand the false positive and detection rates of both tests. A false positive is when a test shows the baby has a high likelihood of having Down’s syndrome when actually the baby does not have it. For the Combined test the national screening committee sets out that the false positive rate must be less than 3% which means that less than 3 out of 100 tests should give a high risk result when in fact the baby does not have Down’s syndrome. For the NIPT the false positive rate is less than 0.1% which means that 1 out of 1000 women will have a false positive result.
The detection rate is the number of babies who truly have Down’s syndrome that are picked up by a test. The detection rate for the Combined test must be over 75% for it to be deemed acceptable by the UK’s National Screening Committee. The NIPT’s detection rate is >98%.
This means that with the NIPT you are more likely to get an accurate result and less likely to miss a diagnosis of Down’s syndrome. It is still not diagnostic but it is closer to being than the Combined test.
Remember that the NIPT is available on the NHS but only to women who come up as high risk on the Combined test. This means that some women will falsely get a low risk result on Combined test and will give birth to a baby with Down’s unexpectedly. Now it depends on the woman’s views on this and their financial position amongst other factors as to whether they choose to go straight to NIPT.
To complicate matters further the NIPT test is a generic name for multiple tests that are available. The different tests are basically organised by different companies; essentially NIPT is the generic name and then this is broken down to branded versions. Women cannot currently buy the test directly from the company performing the analysis; instead they must go through a private clinic. This can be a bit of a minefield for women.
Some NHS hospitals have private wings and these offer NIPT, for example the Kensington wing of Chelsea and Westminster Hospital and Imperial Private Healthcare which is based at Queen Charlotte’s wing of the Hammersmith Hospital. Alternatively, women can use other independent private providers, which they can search for locally across the UK. It can be helpful to know what to ask when looking for a place to carry out NIPT.
- Do they offer a scan first to check dates?
- Who performs the scan and explains the dates – a trained nurse/midwife/doctor or a technician?
- Is the clinic linked to local NHS hospitals or private hospitals so that if there is a problem the woman can be referred for diagnostic testing or other support there?
- What is the cost of the test and what does that include (some add on costs for different options within the test)?
- Which lab does the clinic use? Has the test they offer been validated through an appropriate trial on thousands of women been published in a well-regarded peer-reviewed journal such as the Lancet or the New England Journal of medicine?
- How frequently is the test found to be invalid and have to be repeated and what would repeating it cost?
- How does the clinic communicate the result to the woman? Do they get just the result or some counselling around the result?
- What days and times does the clinic offer the testing?
This website has a link to the different labs that analyse NIPT:
Clearly making a decision which has cost implications and ethical and emotional implications is never easy. I would recommend reading this blog in combination with my last blog: bit.ly/Downsprenatal to support couples trying to make these decisions.
I also recommend they talk to their midwife and/or doctor to get further support.
Originally posted here on PregnaPouch.
Any questions or comments do get in touch @PregnaPouch or PregnaPouch@gmail.com.